Atomic clocks and precision timing devices are an integral part of GPS (Global Positioning System) and GNSS (Global Navigation Satellite System) devices, as well as cellular telephone systems, secure communication protocols, distributed networks, etc. As the sophistication of these systems continues to improve, and data rates continue to increase, the demand for smaller, lower power, more compact timing devices increases as well. Whereas a volumetric footprint of 100 cm3 was once considered more than acceptable for a physics package of an atomic clock, future devices, especially portable devices, will require a timing device ten to one-hundred times smaller than this size.
Efforts are ongoing in the development of such a compact, low-power time/frequency standard (“Chip-Scale Atomic Clock,” or CSAC). There are a number of approaches that may be taken to create a CSAC based on various optical and/or microwave excitation techniques. One such approach, known as Coherent Population Trapping (CPT) uses a single optical source modulated at microwave frequencies to generate the atomic states which can serve as frequency references. Other approaches use microwave excitation and optical interrogation in their operation. At the heart of the CSAC is the hardware assembly to create and interrogate the optical frequency reference known as the physics package. Consider for a moment the physics package 100 for a typical CPT-based CSAC system presented in FIG. 1. As with many prior art systems, the physics package is arranged in a linear fashion with a vertical cavity surface emitting laser (“VCSEL”) 102 spaced significantly apart from a vapor cell 104 and a photodiode which is a detector 106. As can be appreciated by those skilled in the art, the VCSEL 102 generates a beam of light which illuminates and interrogates alkali atoms contained within the vapor cell 104 Multiple window heaters, e g. heaters 108 and 110, are used to heat both vapor cell 104 and VCSEL 102.
Given the size constraints of the optics (lens) 112 in this prior art system, a relatively long path length is required to achieve a beam width of 2 mm or more. A wide beam width (on the order of 2 mm) is required to ensure a sufficient volume of gaseous atoms contained in the vapor cell 104 is illuminated and excited. Other components such as the ND filter 114 and the waveplate 116 only add to the overall physics package size.
There are a number of limitations with many of the atomic clock/physics packages known in the prior art. As noted above, substantially longer path lengths are required to achieve a proper light beam diameter. Longer path lengths equate to larger volumes, which are unsuited for many hand-held portable devices. Further, physical separation of the VCSEL from the vapor cell can require multiple heaters, which may be an inefficient method of heating that consumes an undue amount of power. Therefore, controlling the heating of the VCSEL and vapor cell to ensure consistent and stabilized temperature operations with low operating power can be very difficult.
For these reasons and others, many atomic clock designs now employ a “folded” optics configuration, wherein components are co-located and/or efficiently positioned to reduce volume and power consumption. For example, in many prior art systems the VCSEL and photodiode detector are co-located on a single chip or board. These systems may further reduce path length by utilizing one or more reflectors to redirect light through the vapor cell, thereby minimizing the overall path length. While these systems (atomic clocks) may offer some improvement over more traditional systems, they still have performance issues. Such a configuration may require a custom VCSEL/detector element, and the optical arrangement can result in non-uniform intensity and polarization of light passing through the vapor cell, which can compromise ultimate performance.
Hence there is a need for an atomic clock having a folded optics configuration to address one or more of the drawbacks identified above.